The first edition of this tool covered taxa that were on the federal noxious weed list (7 CFR 360.200) as of 2006 as well as ten additional taxa named by the Federal Seed Act regulations as actionable "with tolerances applicable to their introduction." This revised second edition includes an additional eleven species: Senecio inaequidens DC. and Senecio madagascariensis Poir. were offically adopted as federal noxious weeds on October 5, 2006 (71 FR 58735), and Lygodium flexuosum (L.) Sw. and Lygodium microphyllum (Cav.) R. Br. are two ferns officially adopted as federal noxious weeds on May 3, 2010 (75 FR 23151). As of August, 2010, the final rule is pending on adoption of seven more species (74 FR 27456): Acacia nilotica (L.) Willd. ex Delile, Ageratina riparia (Regel) R. King & H. Robinson, Arctotheca calendula (L.) Levyns, Euphorbia terracina L., Inula britannica L., Onopordum acaulon L., and Onopordum illyricum L.
Thirty-one families are currently represented on the FNW list. Most of the taxa are individual species, but two are species complexes—Rubus fruticosus L. agg. and Salvinia auriculata complex—and one is an infraspecific taxon (Setaria pumila (Poir.) Roem. & Schult. ssp. pallidefusca (Schumach) B. K. Simon). Fact sheets have been prepared for all of these taxa except that the FNW species of seven genera—Aeginetia, Alectra, Cuscuta, Moraea, Orobanche and Phelipanche, and Striga—have been treated together in six "genus-level" fact sheets, for a total of 114 fact sheets.
The interactive family keys include only those FNW taxa that produce seed and fruit disseminules (i.e., angiosperms). Eight taxa are not included in the interactive keys either because they lack angiosperm sexual reproduction altogether or they produce seed only rarely. One group lacking fruits and seeds are the ferns, which reproduce via spores as well as by vegetative means. Reproduction via vegetative disseminules is the primary means of disperal for some non-ferns (three angiosperms and an alga) as well. The eight taxa that have fact sheets but are not in the keys are the terrestial ferns Lygodium flexuosum (L.) Sw. and Lygodium microphyllum (Cav.) R. Br., the aquatic ferns Azolla pinnata R.Br. and the Salvinia auriculata complex, the aquatic angiosperms Hydrilla verticillata (L.f.) Royle and Lagarosiphon major (Ridley) Moss, the sterile angiosperm hybrid Opuntia aurantiaca Lindl., and the alga Caulerpa taxifolia (Vahl) Agardh.